Silicon Photomultipliers (SPMs) are compact, high performance solid state detectors which are of growing importance for nuclear medicine and radiation detection systems. The disclosure presented herein uses SPM detectors and is of particular relevance to medical imaging scanners which use scintiliation radiation detection methods, such as Positron Emission Tomography (PET) including Time-of-Flight (TOF), gamma cameras and Positron Emission Mammography (PEM).
The quality of clinical images in these systems is dependent on many parameters including coincidence resolving time (CRT). Digital SiPMs exhibit a good photon resolution due to the digitization at the cell level of the received signal. However, this requires CMOS integration and affects the cell fill factor due to the inclusion of in-cell logics. Analogue SiPMs do not require internal logic since the output is the analogue sum of the current of each cell. The digitization is external, after the amplification stage. The traditional way to digitize the SiPM output consists in the use of a configurable single-threshold comparator which converts the SiPM output into digital pulses. However, this approach has the primary limitation of using a single threshold. When this is set at a low value, higher peaks of the signal, at high photon rates, are registered with the loss of information of the intensity, i.e. the photon number. For example, in a system with single-photon threshold, any two or three photon-events will be counted as single photon-events.
There is therefore a need to provide for a CRT readout circuit which addresses at least some of the drawbacks of the prior art.